Dynamic analysis of onshore wind turbines including soil–structure interaction

Wind turbines are very sensitive structures. Their slender nature induces a low natural frequencies range, and it combined with spectrum with low frequency of vibration can be very unsafe. Thus, it is of a significant importance to analyze the dynamic behavior of these structures. Considering this, the present study aims to evaluate the dynamic performance of wind turbines via two analytical models, investigating three different conditions and how each one of these will affect in the results. The three aspects are: the influence of the operational condition (parked and rotating), the influence of the SSI and the influence of the rotor velocity. The NREL 5 MW wind turbine will be the source of properties for the tower, nacelle and rotor of the models. The models utilize multi-degree of freedom to represent a horizontal axes onshore wind turbine; moreover, the Euler–Lagrangian approach is used to process the dynamic analysis. The first model was developed using rotating blades modeled as continuous beams. The whole model has eight degrees of freedom. The blades are analyzed as a system of two degrees of freedom, i.e., it is possible to vibrate in the in-plane and out-of-plane directions. They are connected in the center of the tower–nacelle represented by a mass–spring corresponding to a system of two degrees of freedom, that is, it can also have vibrations in the in-plane and out-of-plane directions. The second model has the same characteristics of the first; however, it is included in it a foundation modeled like a spring with bidirectional rotational stiffness. In this way, it is possible, from this model, to study the soil–structure interaction (SSI). Soil stiffness and damping properties were acquired from DNV/Risø standards are used as a comparison. A wind load and an earthquake are used as the source of vibration to the models. The main objective, then, is to evaluate the impact of three different conditions (as previously mentioned) through the dynamic response obtained from the two models. From the results, it is perceived that the responses of the wind turbine in the operating condition are much larger than those in the parked condition; SSI can affect the responses of the tower considerably; however, it does not have a significant effect on the vibrations occurring in the plane of rotation of the blades. Finally, it is perceived that the amplitude of the response increase with the velocity of the rotor, in general.